Film scanners, which are also referred to as telecine, are used to scan cinematographic films and convert them into analogue or digital electrical picture signals. The electrical picture signals can then be stored and duplicated in different formats on suitable media.
During the scanning of the cinematographic films, the film material has to be scanned in such a way that as far as possible no information of the original picture is lost. This requires optimally focusing the scanning device onto the film material.
The films are guided picture by picture or continuously in a guide of the film scanner. The guide has a window, which is also referred to as a film gate. Through the film gate, a film to be scanned is trans-illuminated from one side. A scanning unit of the film scanner is situated on the other side of the film gate. The scanning unit is assigned an optic having adjustable focusing.
In order to set the correct focus position of a film scanner for the scanning of a cinematographic film, generally a picture detail from the centre of a film picture is represented in enlarged fashion. Since not all film pictures of cinematographic films have sharp contours for focusing at every location in the pictures, the focus of the film scanner is set on the basis of the structure of the film grain of the film material. In order to facilitate this, the aperture gain may additionally be increased. The aperture gain is a frequency-dependent signal gain. In this case, the sum of the individual coefficients of a matrix multiplication between an aperture matrix and corresponding pixel values which correspond to the relative position within the matrix is calculated. The electrical signals of the scanned picture have different frequency components that are dependent on the picture content. In this case, fine structures generate high-frequency signal components and coarse structures generate low-frequency signal components. The frequency distribution of the signal of the overall picture can be represented by means of a waveform graph. The waveform graph shows the different frequencies of the picture on the X axis and the respectivie amplitude for the different signal frequencies on the Y axis. The height of the amplitude is in this case a measure of the resolution obtained in the picture and is thus coupled directly to the focus setting. For optimum focusing, the proportion of high-frequency signal components is at a maximum.
Different carrier materials of the films to be scanned flex to different extents or bend differently when they run via the film gate. The bending is caused for example by the heat of the light source, which brings about mechanical stresses on account of the thermal expansion of the film material. If the depth of focus, that is to say the region of sharp imaging of the optic, of a scanning unit is not large enough, it can happen that a region of the film picture to be scanned is already imaged sharply onto the image sensor of the scanning unit, but other regions are not yet imaged sharply. By way of example, it may be that only the edges of the film picture are imaged sharply, but the centre is not, or vice versa. In order to set the focus correctly, the user has to jump back and forth between positions in the different regions of the film picture in order to obtain the optimum setting.
In conventional methods for setting the focus of film scanners, by way of example only picture regions of the horizontal and vertical edges of the film picture are taken into account for a correct setting. In order to simplify the setting, a partial region or detail of the film picture to be scanned is represented in enlarged fashion on a screen of the scanning device. An overview picture may additionally be represented.
In order to check the focus position both in the centre and at the edges, the enlarged partial region has to be shifted in order thus for example successively to check the sharpness at the right-hand edge of the picture, in the centre of the picture and at the left-hand edge of the picture and, if necessary, to readjust the focus position. In the case of setting the focus with the aid of the high-frequency component, in the waveform graph, it is attempted to set the focus such that the amplitude of the high frequency components in the picture regions are maximal both in the centre and at the edges of the film picture.
The sequential sequence during the focusing is. relatively time-consuming. Moreover, the user has in view in each case only the region that is currently to be set.